Liquid-assisted cryogenic cleaning

ABSTRACT

The present invention is directed to the use of a high vapor pressure liquid prior to or simultaneous with cryogenic cleaning to remove contaminants from the surface of substrates requiring precision cleaning such as semiconductors, metal films, or dielectric films. A liquid suitable for use in the present invention preferably has a vapor pressure above 5 kPa and a freezing point below −50° C.

FIELD OF THE INVENTION

[0001] The present invention relates to the use of a liquid, eithersimultaneously or sequentially, with cryogenic cleaning to aid in theremoval of foreign materials, e.g. particles and other contaminants,from semiconductor surfaces, metal films, dielectric films, and othersurfaces requiring precision cleaning.

BACKGROUND OF THE INVENTION

[0002] Cleaning or surface preparation of silicon wafers with or withoutvarious layers of films is very critical in integrated circuitmanufacturing processes. The removal of particles and contaminants fromwafer surfaces is performed at several critical process steps during thefabrication of integrated circuits. At a 0.18 μm technology node, 80 outof 400 steps or 20% of the fabrication sequence is dedicated tocleaning. The challenges of cleaning technology are multiplied by thevaried types of films, topographies, and contaminants to be removed infront-end-of-line (FEOL) and back-end-of-line (BEOL) cleaning processes.Removal of particles is an important part of this cleaning.

[0003] For the defect-free manufacture of integrated circuits, theInternational Technology Roadmap for Semiconductors (ITRS) indicatesthat the critical particle size is half of a DRAM ½ pitch [1]. Thus, atthe 130 nm technology node, the DRAM ½ pitch being 130 nm, the criticalparticles size is 65 nm. Therefore, particles larger than 65 nm sizemust be removed to ensure a defect-free device.

[0004] Such small particles are difficult to remove since the ratio ofthe force of adhesion to removal increases for smaller-sized particles.For submicron particles, the primary force of adhesion of the particlesto a surface is the Van der Waals force. This force depends on the sizeof the particle, the distance of the particle to the substrate surface,and the Hamaker constant. The Van der Waals force for a sphericalparticulate on a flat substrate is given as in equation 1:$\begin{matrix}{F_{ad} = \frac{A_{132}d_{p}}{12Z_{0}^{2}}} & (1)\end{matrix}$

[0005] where A₁₃₂ is the Hamaker constant of the system composed of theparticle, the surface and the intervening medium; d_(p) is the particlediameter; and Z₀ is the distance of the particle from the surface. TheHamaker constant A₁₃₂ for the composite system is given as in equation(2):

A ₁₃₂ =A ₁₂ +A ₃₃ −A ₁₃ −A ₂₃   (2)

[0006] The relationship of the Hamaker constant of two dissimilarmaterials is expressed as the geometric mean of the individual Hamakerconstants as A_(ij)=(A_(ii)*A_(jj))^(1/2) where A_(ii) and A_(jj) arethe Hamaker constants of materials i and j. It is calculatedtheoretically using either the Lifshitz or the London models. TheHamaker constant for particles and surfaces used in integrated circuitmanufacturing processes is given in literature [2, 3] and is less whenthe intervening medium is liquid as compared to air. The Van der Waalsforce, being directly proportional to the Hamaker constant, is thereforereduced when there is a liquid layer between the particle and thesurface.

[0007] In addition to the difficulty in removing small particles fromthe surface, there are various types of organic and metal-organiccontaminants which must be cleaned away. As an example, etching is donein integrated circuit device fabrication processes at a number of stepsboth in FEOL and BEOL to form patterns. The etch is often performed byreactive ion etching (RIE) which generally has a physical and a chemicalcomponent to it. Following this process, the etch residues, which arepolymeric sometimes with metallic contaminants embedded inside thepolymeric matrix, have to be removed. The photoresist film left behindafter the etching also has to be removed prior to the next step in theintegrated device fabrication process. In case of chemical-mechanicalpolishing, the polishing steps may use Cerria, alumina or silicaslurries. After polishing, the slurry and any residues from the slurryadditives need to be cleaned from the wafer surface before the nextlayer of film is deposited. Thus, there is a wide variety of residues,particles and other foreign materials which need to be cleaned both fromthe surface of the wafer as well as inside any etched features.

[0008] The prior art processes use CO₂ or argon cryogenic sprays forremoving foreign materials from surfaces. As examples, see U.S. Pat. No.5,931,721 entitled Aerosol Surface Processing; U.S. Pat. No. 6,036,581entitled Substrate Cleaning Method and Apparatus; U.S. Pat. No.5,853,962 entitled Photoresist and Redeposition Removal Using CarbonDioxide Jet Spray; U.S. Pat. No. 6,203,406 entitled Aerosol SurfaceProcessing; and U.S. Pat. No. 5,775,127 entitled High Dispersion CarbonDioxide Snow Apparatus. In all of the above prior art patents, theforeign material is removed by physical force involving momentumtransfer to the contaminants where the intervening medium betweenparticle and substrate surface is air. Since the force of adhesionbetween the contaminant particles and the substrate is strong, the priorart processes are ineffective for removing small, <0.3 μm, particles.

[0009] U.S. Pat. No. 6,332,470, entitled Aerosol Substrate Cleaner,discloses the use of vapor only or vapor in conjunction with highpressure liquid droplets for cleaning semiconductor substrate.Unfortunately, the liquid impact does not have sufficient momentumtransfer capability as solid CO₂ and will therefore not be as effectivein removing the smaller-sized particles. U.S. Pat. No. 5,908,510,entitled Residue Removal by Supercritical Fluids, discloses the use ofcryogenic aerosol in conjunction with supercritical fluid or liquid CO₂.Since CO₂ is a non-polar molecule, the solvation capability of polarforeign material is significantly reduced. Also, since the liquid orsupercritical CO₂ formation requires high pressure (greater than 75 psifor liquid and 1080 psi for supercritical), the equipment is expensive.

[0010] As such, there remains a need for a more efficient and effectiveremoval process of contaminants, including particles, foreign materials,and chemical residues, from the surfaces of substrates such assemiconductor wafers, metal films, dielectric films, and othersubstrates requiring precision cleaning.

SUMMARY OF THE INVENTION

[0011] The present invention provides for a new and improved process forthe cleaning of substrate surfaces such as semiconductors and metal anddielectric films to remove contaminants.

[0012] The invention uses a high-vapor pressure liquid prior tocryogenic cleaning to reduce the Van der Waals force of adhesion of theforeign material on the surface. The liquid is sprayed onto the surfaceand preferably covers the surface for a short period of time.Preferably, the liquid covers the surface for at least one minute.Following this wetting period, the cryogenic spray is initiated. Thepresence of the liquid will reduce the force of adhesion of thecontaminants on the surface thereby making it easier for the particlesfrom the cryogenic spray to dislodge the contaminants from the surface.The liquid may also remove the bulk water from the surface prior tocryogenic cleaning, such as is used in co-pending U.S. patentapplication Ser. No. 10/215,859 filed on Aug. 9, 2002 and entitled PostCMP Cleaning Using a Combination of Aqueous and Cryogenic Cleaning. Theliquid, if chosen with the correct properties, may also dissolve organiccontaminants from the substrate surface. The high vapor pressure liquidmay be applied simultaneously with the cryogenic cleaning.

BRIEF DESCRIPTION OF THE FIGURES

[0013] Embodiments of the present invention are described with referenceto the Figures in which:

[0014]FIG. 1 is a schematic diagram of the apparatus used in standardCO₂ cryogenic cleaning; and

[0015]FIG. 2 is a graph showing the efficiency of particle removalcompared to particle size for both standard cryogenic cleaning and thepresent liquid-assisted cleaning process.

DETAILED DESCRIPTION

[0016] The invention uses liquids having high vapor pressure to reducethe Van der Waals force between foreign material and a substrate surfacesuch as a semiconductor wafer surface or film surface. The high vaporpressure liquid is sprayed on to the surface of the substrate. It isfollowed with cryogenic cleaning. The initial spraying of liquid willreduce the Van der Waals forces thereby allowing the cryogenic cleaningto more easily remove foreign material from the substrate surface. Ifthe upstream process prior to the cryogenic cleaning is an aqueous basedprocess, as in co-pending U.S. patent application Ser. No. 10/215,859,then the liquid may also remove the bulk water prior to the cryogeniccleaning. Further, the high vapor pressure liquid may act to dissolveorganic contaminants from the surface. A particular high-vapour pressureliquid will be chosen depending on the organic contaminants contained onthe substrate surface. A skilled person in this field will be aware ofthe types of liquids which would dissolve common organic contaminants.

[0017] The liquids suitable for use in the present invention have highvapor pressures. Liquids which are suitable for use include, but are notlimited to, ethanol, acetone, ethanol-acetone mixtures, isopropylalcohol, methanol, methyl formate, methyl iodide, ethyl bromide,acetonitrile, ethyl chloride, pyrrolidine, and tetrahydrofuran. However,any liquid having a high vapor pressure may be used. High vapor pressureliquids will readily evaporate off the surface of the substrate withoutthe need for drying by heating or spinning the substrate. The liquidsalso preferably have low freezing points and are polar in nature. Thelow freezing point of the liquids ensures that any residual liquid lefton the wafer surface at the time of cryogenic cleaning will not freezedue to the drop in wafer temperature that can be attained during thecryogenic cleaning process. The polarity of the liquid aids in thedissolution of organic and inorganic contaminants on the water surface.Preferably, the vapor pressure of the liquid is greater than 5 kPa at25° C., the freezing point of the liquid is below −50° C., and thedipole moment is greater than 1.5 D.

[0018] This process may be used on any substrate surface requiringprecision cleaning. These surfaces include semiconductor surfaces aswell as metal and dielectric films. Therefore, whenever the term“semiconductor”, “metal film”, “dielectric film”, or “wafer” is usedherein, it is intended that the same process may be applied to othersubstrate surfaces. Other surfaces include hard disk media, optics, GaAssubstrates and films in compound semiconductor manufacturing processes.Examples provided herein are not meant to limit the present invention.

[0019] In one embodiment of the present invention, the high-vaporpressure liquid is sprayed onto the surface of a semiconductor wafer ata temperature of 30°-50° C. The liquid may be sprayed either as a thickfilm or as a thin layer. The layer is preferably at least 5-10 Å thick.It is preferably sprayed using a misting nozzle made of Teflon used inwet benches for spraying deionized water onto wafer surfaces. However,any other nozzle used in the art may be employed. The wafer ispreferably covered with the liquid for at least one minute andpreferably up to 10 minutes. The liquid may be applied onto the surfaceonce during this time period or it may be sprayed multiple times toensure that the wafer surface remains wet. As well, the wafer may berotated at approximately 100 rpm while the liquid is sprayed onto it toensure uniform coverage of the wafer surface.

[0020] Following this wetting period, the CO₂ cryogenic spraying isinitiated. Cryogenic spraying processes may use carbon dioxide, argon orother gases and are well known within the art. Any known technique maybe used. The result of the initial high vapor pressure liquidapplication is the reduction of the Hamaker constant and hence the Vander Waals forces. This application lowers the forces of adhesion of theforeign material to the wafer surface and the foreign material is easierto remove from the wafer surface than through the use of only cryogeniccleaning. It also removes bulk water in a prior aqueous cleaningprocess.

[0021] A standard CO₂ cryogenic cleaning process is described in U.S.Pat. No. 5,853,962 which is incorporated herein by reference. As anexample of a typical CO₂ cryogenic cleaning system, reference is made toFIG. 1. The cleaning container 12 provides an ultra clean, enclosed orsealed cleaning zone. Within this cleaning zone is the wafer 1 held on aplaten 2 by vacuum. The platen with wafer is kept at a controlledtemperature of up to 100° C. Liquid CO₂, from a cylinder at roomtemperature and 850 psi, is first passed through a sintered in-linefilter 4 to filter out very small particles from the liquid stream torender the carbon dioxide as pure as possible and reduce contaminants inthe stream. The liquid CO₂ is then made to expand through a smallaperture nozzle, preferably of from 0.05″ to 0.15″ in diameter. Therapid expansion of the liquid causes the temperature to drop resultingin the formation of solid CO₂ snow particles entrained in a gaseous CO₂stream flowing at a rate of approximately 1-3 cubic feet per minute. Thestream of solid and gaseous CO₂ is directed at the wafer surface at anangle of about 30° to about 60°, preferably at an angle of about 45°.The nozzle is preferably positioned at a distance of approximately0.375″ to 0.5″ measured along the line of sight of the nozzle to thewafer surface. During the cleaning process, the platen 2 moves back andforth on track 9 in the y direction while the arm of the cleaning nozzlemoves linearly on the track 10 in the x direction. This results in arastered cleaning pattern on the wafer surface of which the step sizeand scan rate can be pre-set as desired. The humidity in the cleaningchamber is preferably maintained as low as possible, for example <−40°C. dew point. The low humidity is present to prevent the condensationand freezing of water on the wafer surface from the atmosphere duringthe cleaning process which would increase the force of adhesion betweenthe contaminant particles and the wafer surface by forming crystallinebridges between them. The low humidity can be maintained by the flow ofnitrogen or clean dry air.

[0022] As well, throughout the cleaning process, it is important thatthe electrostatic charge in the cleaning chamber be neutralized. This isdone by the bipolar corona ionization bar 5. The system also has apolonium nozzle mounted directly behind the CO₂ nozzle for enhancing thecharge neutralization of the wafer which is mounted on an electricallygrounded platen. The electrostatic charge develops bytriboelectrification due to the flow of CO₂ through the nozzle andacross the wafer surface and is aided by the low humidity maintained inthe cleaning chamber.

[0023] For particulate contaminants, the removal mechanism is primarilyby momentum transfer of the CO₂ cryogenic particles to overcome theforce of adhesion of the contaminant particles on the wafer surface.Once the particles are “loosened”, the drag force of the gaseous CO₂removes it from the surface of the wafer. The cleaning mechanism fororganic film contaminants is by the formation of a thin layer of liquidCO₂ at the interface of the organic contaminant and the surface due tothe impact pressure of the cryogenic CO₂ on the wafer surface. Theliquid CO₂ can then dissolve the organic contaminants and carry it awayfrom the wafer surface.

[0024] Alternatively, the liquid can be applied simultaneously with theCO₂ cryogenic cleaning. In such a case, a second nozzle for spraying theliquid would be mounted in conjunction with a first nozzle used for CO₂cryogenic cleaning. The liquid would preferably be applied in a thinlayer and the CO₂ cryogenic cleaning would continue simultaneously withthe spraying of the liquid onto the substrate.

[0025] As a result of the use of the high vapor pressure liquid, theremoval of particle contaminants by cryogenic cleaning is significantlyimproved. FIG. 2 shows the efficiency of particle removal compared toparticle size for both standard cryogenic cleaning as well asliquid-assisted cryogenic cleaning. Removal of particles having a sizebelow 0.76 μm is significantly improved with the use of the presentliquid assisted CO₂ cryogenic cleaning process rather than standard CO₂cryogenic cleaning. For particle sizes ranging from 0.98 μm to 2.50 μm,there was no significant difference in the removal of particles betweenthe use of the present liquid assisted cryogenic cleaning and thestandard CO₂ cryogenic cleaning process.

[0026] The embodiments and examples of the present application are meantto be illustrative of the present invention and not limiting. Otherembodiments which could be used in the present process would be readilyapparent to a skilled person. It is intended that such embodiments areencompassed within the scope of the present invention.

REFERENCES

[0027] [1]. International Technology Roadmap for Semiconductors 2001Edition.

[0028] [2]. Handbook of Semiconductor Wafer Cleaning Technology Science,Technology and Applications, Edited by Werner Kern, Noyes Publications,1993.

[0029] [3]. Particle Control for Semiconductor Manufacturing, Edited byR. P. Donovan, Marcel Dekker Inc., 1990.

1. A process for the removal of contaminants from a substrate surfacerequiring precision cleaning, comprising the steps of: a) applying ahigh vapor pressure liquid to the substrate surface; and b)cryogenically cleaning the surface of the substrate; to removecontaminants from the substrate surface.
 2. The process of claim 1wherein steps a) and b) are carried out simultaneously.
 3. The processof claim 1 wherein the high vapor pressure liquid has a vapor pressuregreater than about 5 kPa at 25° C., and a freezing point below about−50° C.
 4. The process of claim 3 wherein the high vapor pressure liquidhas a dipole moment of greater than about 1.5 D.
 5. The process of claim1 wherein the high vapor pressure liquid is selected from the groupconsisting of ethanol, acetone, ethanol-acetone mixtures, isopropylalcohol, methanol, methyl formate, methyl iodide, ethyl bromide,acetonitrile, ethyl chloride, pyrrolidine, tetrahydrofuran and mixturesthereof.
 6. The process of claim 1 wherein the substrate is asemiconductor or dielectric film.
 7. The process of claim 1 wherein thehigh vapor pressure liquid remains on the surface in a layer of at least5 Å for between about 1 to 10 minutes prior to the initiation ofcryogenic cleaning.
 8. The process of claim 1 wherein the substrate maybe rotated during the spraying of the high vapor pressure liquid on thesubstrate surface.
 9. The process of claim 1 wherein the contaminantsare less than 0.76 μm in size.
 10. The process of claim 1 wherein thehigh vapor pressure liquid removes bulk water from the surface.
 11. Aprocess of cleaning the surface of a semiconductor or dielectric filmcomprising the steps of: a) spraying a high vapor pressure liquid ontothe surface; b) spraying a liquid CO₂ stream through a nozzle to form agaseous CO₂ stream having solid CO₂ particles, and c) directing saidstream at the surface; thereby removing contaminants from the surface.12. The process of claim 11 wherein the high vapor pressure liquid issprayed as a mist from a nozzle placed behind a CO₂ nozzle and issprayed simultaneously with the CO₂.
 13. The process of claim 11 whereinthe high vapor pressure liquid has a vapor pressure greater than about 5kPa at 25° C., and a freezing point below about −50° C.
 14. The processof claim 13 wherein the high vapor pressure liquid has a dipole momentgreater than about 1.5 D.
 15. The process of claim 11 wherein the highvapor pressure liquid removes bulk water from the surface.
 16. Theprocess of claim 11 wherein the high vapor pressure liquid is selectedfrom the group consisting of ethanol, acetone, isopropyl alcohol,methanol, methyl formate, methyl iodide, ethyl bromide, acetonitrile,ethyl chloride, pyrrolidine, tetrahydrofuran, and mixtures thereof. 17.The process of claim 1 1 where the gaseous CO₂ stream is directed at thesurface at an angle of between 30°-60°.
 18. The process of claim 11wherein the high vapor pressure liquid is sprayed onto the surface asthin layers of at least 5 Å.
 19. The process of claim 11 wherein theparticles are less than about 0.76 μm in size.
 20. A process forcleaning a surface of a semiconductor or dielectric film to removecontaminants having a particle size of about 0.76 μm or less, comprisingthe steps of: a) spraying a high vapor pressure liquid, having a vaporpressure of 5kPa or greater and a freezing point of about −50° C. orless, in thin layers onto the surface; b) leaving the liquid on thesurface for at least one minute, prior to the initiation of cryogeniccleaning of the surface.
 21. A process for cleaning a surface of asemiconductor or dielectric film to remove contaminants having aparticle size of about 0.76 μm or less, comprising the steps of: a)spraying a high vapor pressure liquid, having a vapor pressure of 5 kPaor greater and a freezing point of about −50° C. or less, in thin layersonto the surface; simultaneously with cryogenic cleaning of the surface.